1. Field of the Invention
The present invention relates to a monitor, and more particularly, to a screen brightness controller for a monitor which compensates for variations in brightness with curvature of the screen.
2. Description of the Related Art
A video signal is typically carried on a black level signal that is a predetermined direct current (DC) voltage indicating the brightness of a screen, or a luminance to be scanned to a screen such as by using an electron gun. The screen brightness can be controlled by a user operating a computer.
This conventional brightness control provides control for the entire screen. If the brightness is adjusted to a significantly bright state, a portion to be black on a screen appears gray, and a portion to be gray appears black. Also, if the brightness is adjusted to an excessively dark state, black appears gray and gray appears black, causing an abnormal display of the screen.
FIG. 1 is a block diagram of a conventional screen brightness controller in a monitor. Referring to FIG. 1, the brightness controller 1 includes an amplifier 2, a comparator 3, a clamp capacitor 4 and a switch 5. The amplifier 2 has a non-inverting input port for receiving a video input signal Vin having a video signal carried on a black level signal and an inverting input port to which the clamp capacitor 4 is connected. The amplifier 2 provides as an output a video output signal Vout which is amplified by a difference in voltage between the two input ports.
The comparator 3 receives the video output signal Vout and a brightness control voltage B for controlling the brightness of a monitor screen. The comparator 3 compares the black level voltage of the video output signal Vout with the brightness control voltage B.
The switch 5 is connected between the output port of the comparator 3 and the inverting input port of the amplifier 2. While the video output signal Vout is flied back, the switch 5 is switched on (closed) and the clamp capacitor 4 is charged or discharged, thus changing the voltage at the inverting input port of the amplifier 2.
The configuration of FIG. 1 is used on the brightness controller 1 because an image is carried on the black level signal when the video output signal Vout is horizontally scanned, but the video output signal Vout has only the black level signal during fly-back.
The operation of the monitor screen brightness controller 1 of FIG. 1 will now be described. When the switch 5 is switched on (closed), the comparator 3 compares the black level signal of the video output signal Vout with the brightness control voltage B. If the black level signal is greater than the control voltage B, the charges stored in the clamp capacitor 4 are discharged to the amplifier 2 by a current output from the comparator 3. Thus, the voltage of the clamp capacitor 4 decreases, and that of the non-inverting input port of the amplifier 2 increases, and, consequently, the black level signal of the video output signal Vout is lowered. The video output signal Vout is again input to the comparator 3, and the aforementioned operations are repeated until the black level signal of the video output signal Vout is equal to the brightness control voltage B.
On the other hand, if the black level signal of the video output signal Vout is smaller than the brightness control voltage B, the clamp capacitor 4 is charged by the current output from the comparator 3. Therefore, the voltage of the clamp capacitor 4 increases, and the voltage of the other input port of the amplifier 2 decreases. As a result, the black level signal of the video output signal Vout is increased. The video output signal Vout is again input to the comparator 3, and the aforementioned operations are repeated until the black level signal of the video output signal Vout becomes the same as the brightness control voltage B. A user can change the brightness of a screen to a desired level by controlling the brightness control voltage B.
FIG. 2 is a schematic view of a conventional cathode ray picture tube. Referring to FIG. 2, an electron gun 21 scans an electron beam over a screen 22 to display an image on the screen 22. The image on the screen 22 is controlled by, among other factors, the intensity of the electron beam as it strikes the screen 22 at each position. The intensity of the beam is controlled by a video output signal Vout, received by the electron gun.
The surface of the screen 22 is typically relatively flat compared to the radius centered at the output of the gun 21 (shown in dashed lines), such that distances between the electron gun 21 and the screen 22 vary over the scanning period. That is, a distance d between the center of the screen 22 and the electron gun 21 is less than a distance d+.DELTA.d between the edge of the screen 22 and the electron gun 21. This variation in distance causes a variation in path length of the beam with respect to positions on the screen. This can result in reduced image intensity at the edges of the screen.
FIG. 3 is a schematic plot illustrating an exemplary video output signal Vout shown in FIG. 2 with respect to time. Referring to FIG. 3, the video output signal Vout includes a black level signal having a constant amplitude.
When the video output signal Vout is horizontally scanned to a screen via the electron gun, the black level signal scanned to the edges of the screen appears smaller than that scanned to the center of the screen, since the scan distance, i.e., the distance between the screen and the electron gun, is not constant. As a result, the edges of the screen 22 can appear darker than the center of the screen.